1. Field of the Invention
The present invention relates to a structure of a photodetector device mainly employed for optical communications.
2. Description of Related Art
Research and development have recently been intensely conducted to increase a modulation frequency in order to transmit a larger volume of information in optical communications using optical fibers. Accordingly, research aimed at the increase (to no less than 20 Gb/s) of operation speed of photodetector devices (for example, photodiodes) for detecting the modulated signal light has been actively conducted.
FIG. 10 shows an example of a typical conventional structure of such photodiode. Thus, the photodiode shown in the figure has a pin-type structure with a monolayer configuration of an end surface incidence type in which an incident light I0 from an end surface (side surface) is received. In this FIG. 10, the reference numeral 1 stands for a semiinsulating substrate composed of InP (indium-phosphorus) having semiinsulating properties, 3 for an n-type blocking layer composed of n+-type InP for preventing the injection of positive holes from the outside, 5 for a light-absorbing layer (i-type or intrinsic semiconductor layer) composed of i-type InGaAs (indium-gallium-arsenic) for absorption of light and generation of electron-hole pairs, 7 for a p-type blocking layer composed of p+-type InP for preventing the injection of electrons from the outside, and 9 for a contact layer composed of p++-type InGaAs for the formation of ohmic contact with an electrode. Furthermore, the reference numeral 11 stands for a first electrode electrically connected to the n-type blocking layer 3 and having a configuration formed by successive lamination of a gold-germanium alloy (AuGe), nickel (Ni), and gold (Au) (that is, a AuGe/Ni/Au laminated configuration) on the end portion of the n-type blocking layer 3. The reference numeral 13 stands for a second electrode electrically connected to the contact layer 9 and having a configuration formed by successive lamination of titanium (Ti), platinum (Pt), and gold (Au) (that is, a Ti/Pt/Au laminated configuration) on the contact layer 9. The reference character 5 stands for an output signal terminal, G for a ground terminal, and V for a power source terminal.
When such photodiode is driven, a voltage of about −3˜−5 V (volts) is applied to the power source terminal V and the ground terminal G is grounded. Further, d stands for a thickness of the light-absorbing layer 5. The typical thickness of photodiodes of this type is 0.4˜1.0 μm. Furthermore, characters h, e, and E stand for holes, electrons, and electric field, respectively.
Electron-hole pairs are generated by a signal light I0 (wavelength λ=1.55 μm) incident onto the end surface of the light-absorbing layer 5 of the photodiode and absorbed by the light-absorbing layer 5. Under the effect of electric field E applied in the light-absorbing layer 5, electrons drift toward the n-type blocking layer 3 and the holes drift toward the p-type blocking layer 7. The holes that reached the p-type blocking layer 7, which is in the upper portion, and the electrons that reached the n-type blocking layer 3, which is in the lower portion, are led out to the outside as current from the output signal terminal S.
A structure of an upper surface incidence type in which the incident light I0 is received from the upper surface, as shown in FIG. 11, is another example of the typical conventional structure of the photodiodes of this type. The photodetector device of such structure has a photodiode unit of a pin-type with a monolayer configuration. In FIG. 11, the reference numeral 1 stands for a semiinsulating substrate composed of InP having semiinsulating properties, 3 for an n-type blocking layer composed of n+-type InP for preventing the injection of positive holes from the outside, 5 for a light-absorbing layer (i-type or intrinsic semiconductor layer) composed of i-type InGaAs for absorption of light and generation of electron-hole pairs, 7 stands for a p-type blocking layer composed of p+-type InP for preventing the injection of electrons from the outside, and 9 for a contact layer composed of p++-type InGaAs for establishing an ohmic contact with an electrode.
The reference numeral 11 stands for a first electrode electrically connected to the n-type blocking layer 3 and having a configuration formed by successive lamination of a gold-germanium alloy (AuGe), nickel (Ni), and gold (Au) (that is, a AuGe/Ni/Au laminated configuration) on the end portion of the n-type blocking layer 3. The reference numeral 13 stands for a second electrode electrically connected to the contact layer 9 and having a configuration formed by successive lamination of titanium (Ti), platinum (Pt), and gold (Au) (that is, a Ti/Pt/Au laminated configuration) on the contact layer 9. The reference character S stands for an output signal terminal, G for a ground terminal, and V for a power source terminal. An opening 13a is provided in the second electrode 13 in the region corresponding to the incident light photodetection region.
During driving of such photodiode, a voltage of about −3˜−5 V (volts) is applied to the power source terminal V and the ground terminal G is grounded. Further, d stands for a thickness of the light-absorbing layer 5. The typical thickness of the photodiode of this type is 0.4˜1.0 μm. Furthermore, characters h, e, and S stand for holes, electrons, and electric field, respectively.
The signal light I0 (wavelength λ=1.55 μm) incident onto photodiode from the upper surface is absorbed by the light-absorbing layer 5 and electron-hole pairs are thus generated. The electrons and holes drift toward the n-type blocking layer 3 and p-type blocking layer 7, respectively, by the electric field E applied in the light-absorbing layer 5. The holes that reached the upper p-type blocking layer 7 and the electrons that reached the lower n-type blocking layer 3 are led out to the outside as current from the output signal terminal S.
The pin-type photodiode of an end surface incidence type having a photodiode unit of a monolayer configuration has a response to the signal light modulated by a modulation frequency of 10˜40 Gb/s. However, in order to obtain an even higher response in the diodes of this type, the thickness d of the light-absorbing layer 5 is preferably decreased to shorten the maximum travel time τtr of the carriers. However, if the thickness d is made too small, the capacity C of the photodiode is increased, the CR time constant τCR which is another factor affecting the response is increased, and the response is slowed. Furthermore, because the amount of incident light is also decreased, the sensitivity dropped. In addition, the incidence position of the signal light incident from the outside was difficult to align on the element end surface, thereby limiting the increase in response rate.
Moreover, in the above-described photodiode with a pin-type structure of an upper surface incidence type having a photodiode unit of a monolayer configuration, the surface area of the light-receiving surface had to be enlarged. Therefore, the capacitance C generated in the element was increased and the CR time constant τCR, was increased. As a result, the photodiode demonstrated a response only to a signal light modulated with a modulation frequency of about 20 Gb/s.
It is an object of the present invention to resolve the above-described problems and to provide a photodiode that can response (operate) at a rate higher than that of the above-described conventional diodes.